PROJECT SUMMARY A major regulator of synaptic function is local protein synthesis. Deep RNA sequencing has revealed that there are thousands of dendritically localized mRNAs. Local translation of selected mRNAs in dendrites provides a fast, adaptive mechanism for the experience-dependent formation of new synapses or the stability of pre- existing connections. This plasticity underlies changes in neuronal network dynamics and is therefore thought to be the foundation of learning and memory. Altered protein synthesis and synaptic plasticity are associated with a variety of neurodevelopmental disorders. However, the pathways that regulate the dendritic proteome are not well understood. Protein synthesis in dendrites requires precise regulation of local mRNA stability and translation. A great amount of prior research has addressed the pathways that regulate translational derepression in dendrites. However, the mechanisms that control mRNA levels during synaptic function have not been demonstrated. We have recently shown that intra-axonal translation coupled to the mRNA-degradation pathway `Nonsense Mediated mRNA Decay' (NMD) controls a switch in receptor expression and thereby regulates axon guidance; indicating that mRNA turnover is a key player in local protein synthesis. Currently, it is not known whether mRNA stability in dendrites contribute to the regulation of synaptic plasticity. The goal of this application is to understand the contribution of intra-dendritic translation coupled to mRNA- degradation pathway NMD to synaptic plasticity and cognitive performance. The synaptic plasticity protein Arc is a known target of NMD-mediated mRNA degradation, serving to limit the amount of Arc in dendrites. We have found that, in addition to Arc, NMD limits the amount of various other proteins involved in GluR1 regulation, which is essential for modulation of synaptic strength. Based on the published literature and our preliminary studies, we hypothesize that local NMD is as essential for synaptic function as it is for axon guidance. To test this hypothesis, we propose to determine whether NMD: 1) locally functions in dendrites; 2) promotes synaptic strength by restricting either internalization or translational repression of GluR1; 3) plays a role in different forms of synaptic plasticity (e.g. LTP and LTD); 4) is required for learning and memory. We will use a combination of techniques including a novel microfluidic device to uniquely study synaptic events, an inducible-genetic mouse model, electrophysiology and behavioral assays. Although NMD is the only RNA regulatory pathway linked to numerous neurocognitive disorders, it represents a relatively unexplored mechanism for regulating synaptic function. The successful completion of this research will provide a coherent view of local proteome dynamics in synaptic plasticity and might be valuable for providing new insights into the mechanisms of synaptic dysfunction and neurocognitive diseases.